It is often useful for processes in molecular biology, such as size determination, sequence determination, or the like, to generate sets of related polynucleotides that differ in length by regular amounts, Sanger et al, Proc. Natl. Acad. Sci., 74: 5463-5467 (1977); Brenner, U.S. Pat. No. 5,552,278; Hyman, U.S. Pat. No. 5,840,757; Hartley, U.S. Pat. No. 6,680,378; Hu et al, U.S. Pat. No. 6,924,098; Carlson et al, U.S. Pat. No. 5,316,908; and the like.